A Liquid Crystal Display (LCD) device is a display device that controls the transmission/shielding (On/Off of display) of light by controlling the alignment of liquid crystal molecules having birefringence. Generally, the liquid crystal molecules are initially aligned, and then the alignment is changed by the application of voltage. Examples of the technique of initially aligning the liquid crystal molecules between substrates include a method of arranging an alignment film which is subjected to an alignment treatment such as rubbing method or photo-alignment method (photo-alignment technique) on the surface of the substrate that contacts the liquid crystal layer.
Such a liquid crystal display device has characteristics of low power consumption, a thin shape, and a high display performance. The liquid crystal display device is used for large-screen television, mobile phones, and the like, and has become indispensable in daily lives and businesses.
Incidentally, regarding alignment control of liquid crystal molecules, various display modes having diverse characteristics according to the arrangement of electrodes or driving method have been developed. For example, in a liquid crystal display device employing a display mode such as an In-Plane Switching (IPS) mode or a Fringe Field Switching (FFS) mode using a horizontal alignment film in which liquid crystal molecules are aligned approximately in a horizontal direction when voltage is not applied, the liquid crystal display device can exhibit the wide viewing angle characteristic.
In those various display modes, the method of alignment treatment for a substrate that determines the initial alignment of liquid crystal molecules is important for the display quality. However, if an alignment film is formed by rubbing, foreign substances are generated, or the alignment film is scratched. Accordingly, the display quality or the yield deteriorates in some cases. As a solution to the problem, weakening the rubbing strength can be considered. However, in this case, an alignment regulating force of the alignment film deteriorates, and this leads to a concern that the alignment may be disordered, or an image sticking may occur during liquid crystal display. The “image sticking” is a phenomenon in which when an image is displayed for a long time and then replaced with another image, the image having been displayed before the image replacement remains and is displayed.
On the other hand, the alignment treatment using the photo-alignment technique can impart the alignment regulating force to the film without contacting the film. Accordingly, the alignment film is not scratched, or foreign substances are not generated. However, the alignment film itself formed by the photo-alignment technique (hereinafter, also called a “photo-alignment film”) has weak alignment regulating force. Therefore, in this film, disordered alignment or image sticking is observed in some cases similarly to the alignment film formed by weakened rubbing strength, so the film has not been commercialized.
In consideration of the above problems, a method of suppressing hysteresis in electro-optic characteristics, that is, a method of stabilizing liquid crystal arrangement by adding a photopolymerizable monomer to liquid crystals to form a liquid crystal display panel and then irradiating the panel with light to form a polymer has been proposed (for example, refer to Non-Patent Literatures 1 and 2).
Such a technique for stabilizing the alignment by using a polymer including a photopolymerizable monomer is also called a “Polymer Sustained Alignment (PSA) technique”.